A. Field of the Invention
B. Description of the Related Art
The present invention contemplates a new and improved process for reducing the surface roughness of thin films which is simple in design, effective in use, and overcomes the foregoing difficulties and others while providing better and more advantageous overall results.
There are many commercial applications for thin films and, in particular, multilayer films. One particularly promising application is the use of these films in fiber-optic networks. Multilayered films are used in Dense Wavelength Division Multiplexers/Demultiplexers (DWDM) systems which enable information to be delivered inside the fiber optic cables at multiple wavelengths.
The ability to transmit data via fiber optic cables has become of increasing importance in this technological age. At the present time, the installation of a world-wide fiber-optic network is in progress which will be capable of handling levels of data transmission inconceivable only several years ago. As a result of this network, the Internet is less than half a decade away from being a more useful tool than the computers which navigate it. As the biggest technological revolution in the history of modern civilization progresses, advanced high performance coatings have emerged as the enabling technology. The ability to control transmission and reflection of selected wavelengths of light has enabled existing fiber to accommodate the increase in bandwidth which will be required over the next 3-5 years.
Dense Wavelength Division Multiplexers/Demultiplexer (DWDM) systems enable information to be delivered inside fiber-optic cables at multiples wavelengths. The increase in the bandwidth is limited only by the number of wavelengths which can be superimposed on the fiber. Current state-of-the-art DWDMs can multiplex/demultiplex approximately 130+ channels. Ultimately more than 1000 channels will be possible. During transmission, information is packaged within phase modulated carriers at specific wavelengths and superimposed (multiplexing) on the fiber. During reception, the carriers must be separated (demultiplexing). Optical component technology such as DWDMs are critical to achieve bandwidth necessary for future interactive services such as “video on demand,” and have prompted multi-billion dollar strategic acquisitions such as OCLI, NetOptix, and XROS.
The most widely used technology for multiplexing and demultiplexing in DWDM systems is thin film-based. Multilayered thin dielectric coatings are comprised of 150-200 layers with individual optical layer thickness equal to multiples of ¼ of the wavelength to be transmitted (known as dielectric interference filters). A collection of such filters coupled together, each differing slightly in design to allow light transmission of different wavelengths, and “connected” to a fiber-optic cable enables the multiplexing (superposition) and demultiplexing (separation) of multiple wavelengths of laser light containing digital information.
Current thin film multiplexers and demultiplexers can handle up to 40 different wavelengths but several manufacturers have announced 80 channel versions in year 2000. With current state-of-the-art deposition processes used for DWDM, 80 channel multiplexers will approach the limit of the technology. Theoretical thin film filter designs exist with Full Width at Half Maximum (FWHM) of less than 0.1 nm. Such a filter would enable multiplexers capable of handling more than 1000 channels.
Thin film coatings designed to permit light transmission/reflection over narrow (0.1-25 nm) and broad (>25 nm) pass bands are typically comprised of multiple layers of two or more optically matched materials of “high” and “low” indices of refraction. The individual layer thickness and number of layers will ultimately define the optical performance of the filter. Typical narrow band filters (called “high performance”) may have more than 100 individual layers.
High performance dielectric thin film optical filters are produced in volume for state-of-the-art multiplexers and demultiplexers used in DWDM systems. These filters are produced with materials such as SiO2 and Ta2O5 deposited with processes such as ion beam sputter deposition (ISBD) and ion-assisted deposition (IAD). Filters produced with these processes are stable under adverse environmental conditions but lack necessary thickness and roughness control to multiplex/demultiplex more than 80 channels in the desired wavelength range. This is primarily due to excessive roll off of the filter which leads to full width at half maximum (FWHM) of greater than 2 nm (250 GHz).
Surface roughness at interfaces and thickness control are critical factors in determining the performance of a narrow bandpass filter. State-of-the-art filters will incorporate interfacial roughness which increases exponentially with layer number and is ultimately greater than 10% of the layer thickness. Furthermore, in-situ optical and physical thickness monitoring techniques are accurate to within 0.5%. This level of layer control has enabled narrow bandpass filters on the order of 1 nm FWHM (125 GHz).
Surface roughness reduction and interface smoothing by ion bombardment has been examined extensively for multilayered films designed for x-ray reflectors. In that collection of work it was observed that, by ion polishing the film surfaces using Ar+ or O+ ions accelerated from an ion source, surface roughness (Ra) was reduced by a factor of 2. It was also observed that deposition of a thin amorphous carbon (C) layer at each interface, between layers of multilayered reflectors, was successful at reducing interface roughness.
Diamond-like carbon (DLC) produced with plasma-based processes such as ion beam deposition (IBD) and plasma enhanced chemical vapor deposition (PECVD) is a smooth, amorphous and virtually lossless carbon coating. Since the material can be made more than 99% transmissive in the infrared (IR, 800 nm-15 μm) and has a refractive index of n=2.0, it is commonly used for many IR window applications. Intrinsic stress is compressive and can be quite high ultimately leading to cracking and delamination in coatings greater than 3.0 μm thick. These coatings are also used for anti-abrasion since DLC produced with ion beam processes such as IBD or PECVD is a very hard low surface roughness amorphous carbon coating.
Diamond-like carbon (DLC), and other forms of amorphous carbon, can be stripped from substrates by exposing the surface to an energetic (>50V) oxygen plasma. The energetic oxygen ions react chemically with the carbon surface to form carbon monoxide (CO). The vapor pressure of CO is high enough, at the vacuum level at which this process is performed (˜10−2 torr), that the CO molecules evaporate from the surface. The freshly exposed surface carbon then reacts with the plasma and the process continues until the oxygen plasma is extinguished or no amorphous carbon remains.